The need for ongoing exploration and production of fossil fuels is fundamental to the existence of industrialized societies. As fossil fuel reservoirs underground are depleted, there is much activity in offshore exploration and production.
The tension leg platform (TLP) has become an important part of this offshore production. Essentially, the TLP is a floating platform which is connected to the sea floor through a plurality of relatively long and thin tension bearing elements referred to a tethers. Flexible joints are provided at both the lower and upper end of each tether, which permits the platform to move relative to the sea floor under the action of wave, wind and tide.
In one design, the upper end of the tether is provided with a flex joint and a CLB housing. The flex joint is connected between the tether itself and the inside of the CLB housing, which permits the tether to pivot relative to the CLB housing. A tether extension will provide a socket for the top end of the tether to maintain the pivot point of the tether along the central axis of symmetry of the CLB housing and also maintain the elastomeric elements of the flex joint in compression to lengthen their service life.
The CLB housing, in turn, is positioned within a tether conduit which forms part of the floating platform. The tether conduit defines an inner cylindrical surface, in which is the CLB housing, flex joint and upper end of the tether. Radial bearings are provided between the CLB housing and the inner surface of the tether conduit to transfer side loads between the conduit and CLB housing. The tether conduit, and thus the floating platform, can move vertically relative to the CLB housing and tether by permitting the housing to slide along the inner cylindrical surface of the tether conduit. Due to the flex joint between the upper tether end and the CLB housing, the platform can also pivot relative to the tether about the tether pivot point.
As can readily be understood, buoyancy is only achieved by displacing water with a water tight structure whose weight is less than the water displaced. By providing a seal between the CLB housing and inner cylindrical surface of the tether conduit, the voids above the CLB housing within the tether conduit can be kept dry to enhance the buoyancy of the platform. Therefore, in the past, seals have been designed and installed in TLP environments to perform this function. Typically, two seals are provided for each CLB housing, one near the bottom of the CLB housing below the radial bearings and one near the top of the CLB housing above the radial bearings. In the prior design, the seals consisted of inflatable donut shaped elastomeric bladders which were inflated by air to press against the CLB housing and inner surface of the tether conduit. The seals were designed to roll along the inner cylindrical surface of the conduit to compensate for any axial motion between the CLB housing and tether conduit. Anticipated relative axial motion was relatively small, about 95 mm, and a rolling type seal was believed to be adequate.
However, over time, many of these tube seals failed. While none of the failures were major operational problems, but merely relative slow leakage of a fluid into the sealed areas rather than rapid flooding, the seal failures contributed to three abnormal operating conditions.
In certain cases, construction debris and tools fell into the tether conduit and were allowed to remain on the operational tube seals. This resulted in puncturing some of the tube seals. Construction debris included such items as welding slag, metal cuttings and shavings, as well as large pieces of metal. That debris had lodged between the tube seal and its mounting structure, as well as between the tube seal and conduit wall.
Another problem arose in incorrect installation in a displaced configuration which precluded the seal's function for its designed purpose of accommodating vertical motion through rolling of the seal. Vertical displacements of the conduit relative to the CLB housing resulted in tears at the root area of several seals due to high stress. The inter- leaved layers of rubber and woven fabric in that area of the seals separated and tore.
The prior designs anticipated no yaw condition, i.e. where the CLB housing would become canted within the tether conduit, nor to accommodate torsional motion between the vertical axis of the tether and the vertical axis of the tether conduit. However, such motion occurred, which led to rubber to fabric bond failures within the seal reinforcing material as well as abrasion of the seal itself. The rubber tube fabric bond failures were further aided and abetted by exposure of the reinforcing fabric to the seawater. Cuts and abrasions in the tube seal exposed the fabric reinforcement to seawater. The fabric then absorbed and wicked water between the interleaved layers of fabric and cord, thereby providing an additional means of rubber to fabric bond failure.
A need therefore exists for an improved sealing procedure for the seal between a CLB housing and a tether conduit to maintain the buoyancy provided by proper operation of that seal, preferably for the lifetime operation of the platform of 20-30 years.